1. Field of the Invention
The present invention relates to an optical module used in a transmitter for transmitting optical signals, such as a semiconductor laser module or the like, and a method of making the same. The present invention particularly relates to an optical module suitable for use in a light-signal transmission in a wavelength division multiplexing (WDM) communication system and a method of making the same.
2. Discussion of the Background
Generally, the field of dense WDM requires optical transmitters to produce light-signals at stable wavelengths for a long time. To accomplish this, it has been developed an optical module that includes a wavelength monitor located in the package thereof. One of the prior art optical modules including the wavelength monitor is disclosed, for example, in Japanese Patent Laid-Open Application No. Hei 12-56185.
Referring first to FIG. 20, there is shown an optical module constructed according to the prior art and having a wavelength monitor. The optical module includes a laser diode 50 for outputting a laser beam with a predetermined wavelength; an optical fiber 51 optically coupled with the laser diode 50 and adapted to externally deliver the laser beam outputted from the laser diode 50 at its front end face (right side as viewed in FIG. 20); an optical filter 52 having its cutoff wavelength substantially equal to the lasing wavelength of the laser diode 50; a beam splitter 53 including a half mirror for dividing a monitoring laser beam outputted from the laser diode 50 at its back end face (left side as viewed in FIG. 20) into two laser beam components; a first photodiode for receiving one of the two laser beam components divided by the beam splitter 53 after it has passed through the optical filter; a second photodiode 55 for receiving the other laser beam component from the beam splitter 53; a Peltier module 56 for regulating the temperature in the laser diode 50; and a control unit 57 for controlling the Peltier module 50 to control the wavelength in the laser diode 50, based on PD currents outputted from the first and second photodiodes 54, 55.
Between the laser diode 50 and the optical fiber 51 is disposed a condensing lens 58 for coupling the laser beam from the front end face of the laser diode 50 with the optical fiber 51. Between the laser diode 50 and the beam splitter 53 is also disposed a collimating lens 59 for collimating the laser beam outputted from the back end face of the laser diode 50.
The laser diode 50, condensing lens 58 and collimating lens 59 are fixedly mounted on an LD carrier 60. The first and second photodiodes 54, 55 are fixedly mounted on first and second PD carriers 61, 62, respectively.
The beam splitter 53, optical filter 52 and first and second PD carriers 62 are fixedly mounted on a metallic base plate 63 that is fixedly mounted on the surface of the LD carrier 60. The LD carrier 60 is fixedly mounted on the Peltier module 56.
The laser diode 50, beam splitter 53, optical filter 52, condensing lens 58, collimating lens 59, LD carrier 60, first PD carrier 61, second PD carrier 62, metallic base plate 63 and Peltier module 56 are housed within a package 64. The tip end of the optical fiber 51 is held by a ferrule 65 that is fixedly mounted on the side of the package 64 through a sleeve 66.
The laser beam outputted from the front end face of the laser diode 50 is condensed by the condensing lens 58 and then enters the optical fiber 51 held by the ferrule 65 before it is externally delivered therefrom.
On the other hand, the laser beam outputted from the back end face of the laser diode 50 is collimated by the collimating lens 59 and then enters the beam splitter 53 wherein the laser beam is divided into two laser beam components, directed to a Z-axis direction (or direction of transmission) and an X-axis direction, (or direction of reflection) perpendicular to the Z-axis direction. The laser beam component directed to the Z-axis direction is subjected to wavelength filtering by the optical filter 52, and is then received by the first photodiode 54 while the laser beam component directed to the X-axis direction is received by the second photodiode 55. PD currents outputted from the first and second photodiodes 54, 55 enter the control unit 57 that, based on the received PD currents, controls the temperature in the Peltier module 56 to control the wavelength in the laser diode 50.
In the conventional optical module that contains the wavelength monitor, the first and second photodiodes 54, 55 are for respectively receiving the divided laser beam components and cannot be arranged in the same plane since the laser beam is divided by the half-mirror type beam splitter 53 into such two laser beam components directed to the Z-axis direction and X-axis direction perpendicular to the Z-axis direction. Thus, the prior art device, as recognized by the present inventor, must use two separate PD carriers 61 and 62 for fixedly supporting the first and second photodiodes 54, 55. As a result, the number of parts increases to raise the manufacturing cost.
The half-mirror type beam splitter 53 has a wavelength dependency since the laser beam is divided into two laser beam components, one reflected by the mirror and one transmitted the mirror. The dense WDM particularly requires high-precision wavelength control of laser beam. As recognized by the present inventors, the wavelength dependency on the laser beam components divided by the half mirror may lead to error in the wavelength control.
Each of the two PD carriers 61 and 62 must independently be subjected to optical aligning. As a result, the number of manufacturing steps increases to prolong the manufacturing time.
Moreover, the wavelength characteristic of the optical filter 52 is variable depending on the angle of incident light. Notwithstanding, the prior art device fixedly mounts the optical filter 52 on the metallic base plate 63 and incorporates the metallic base plate 63 into the optical module before the wavelength monitor unit is completed in assembly. In such a procedure, the set angle of incident light relative to the optical filter 52 can not be changed after the wavelength monitor unit has been incorporated into the optical module. This is disadvantageous in that any desired wavelength characteristic of the optical filter 52 cannot be provided due to failures in the position and angle of the wavelength monitor unit in the optical filter or depending on the position and angle of the wavelength monitor unit when it has been incorporated into the optical filter. This reduces yields for optical module.
In addition, the conventional optical module is not readily scalable. The lack of scalability is due to an increase in the parts needed to ensure a reproducible and obstruction-free optical paths from the laser diode to the respective photodiodes.
One aspect of the present invention is to address the above-identified and other deficiencies and limitations associated with conventional optical module devices and optical transmission methods.
In contrast to the prior art, the present invention provides an optical module that can be produced with reduced manufacturing cost and time and that can be reduced in size with its improved wavelength stability for the laser beam, and a method of making such an optical module.
The present invention also provides an optical module that can be adjusted relating to its angle of incident light relative to the optical filter to provide a predetermined wavelength characteristic for improving yields, after assembled, and a method of making such an optical module.
The present invention provides an optical module that includes
a light-emitting device for outputting a laser beam;
an optical fiber for receiving and externally delivering the laser beam outputted from the light-emitting device at one facet;
a beam splitter for dividing a monitoring laser beam outputted from the light-emitting device at the other facet into two laser beam components, the two laser beam components being inclined relative to the optical axis within a predetermined angle less than 90 degrees;
a first photo detector for receiving one of the two laser beam components divided by the beam splitter;
a second photo detector for receiving the other laser beam components;
an optical filter disposed between either of the first or second photo detector and the beam splitter and for permitting only a laser beam having a predetermined wavelength range to transmit therethrough; and
a mount member on that both the first and second photo detectors are mounted.
The present invention further provides a method of making an optical module, including the steps of:
(1) fixing a light-emitting device for outputting a laser beam;
(2) fixing a beam splitter for dividing a monitoring laser beam outputted from the light-emitting device at the other facet into two laser beam components, said two laser beam components being inclined relative to the optical axis within a predetermined angle less than 90 degrees;
(3) fixing on a mount member a first photo detector for receiving one of the two laser beam components divided by the beam splitter and a second photo detector for receiving the other laser beam components;
(4) fixing an optical filter disposed between either of the first or second photo detector and the beam splitter and for permitting only a laser beam having a predetermined wavelength range to transmit therethrough; and
(5) fixing an optical fiber for receiving and externally delivering the laser beam outputted from the light-emitting device at one facet.